1. Field of the Invention
The present invention relates to an electrochemical cell with a long operating life. In particular, this invention relates to batteries with a controlled release anode to achieve an extended operating life.
2. Brief Description of the Prior Art
Metal-air batteries produce electricity by the electrochemical coupling of a reactive metallic anode to an air cathode through a suitable electrolyte in a cell. The air cathode is typically a sheet-like member, having one surface exposed to the atmosphere and another surface exposed to the aqueous electrolyte of the cell. During cell operation oxygen is reduced within the cathode while anode metal is oxidized, providing a usable electric current flow through an external circuit connected between the anode and cathode. The air cathode must be permeable to air but substantially impermeable to aqueous electrolyte, and must incorporate an electrically conductive element to which the external circuit can be connected. Commercial air cathodes are commonly constituted of active carbon (with or without an added dissociation-promoting catalyst) in association with a finely divided hydrophobic polymeric material and incorporating a metal screen as the conductive element. A variety of anode metals have been used or proposed; among them, zinc, lithium, alloys of aluminum and alloys of magnesium are considered especially advantageous for particular applications, owing to their low cost, light weight, and ability to function as anodes in metal-air battery using a variety of electrolytes.
A typical aluminum-air cell comprises a body of aqueous electrolyte, a sheet-like air cathode having one surface exposed to the electrolyte and the other surface exposed to air, and an aluminum alloy anode member (e.g. a flat plate) immersed in the electrolyte in facing spaced relation to the first-mentioned cathode surface. Aqueous electrolytes for metal-air batteries consist of two basic types, namely a neutral-pH electrolyte and a highly alkaline electrolyte. The neutral-pH electrolyte usually contains halide salts and, because of its relatively low electrical conductivity and the virtual insolubility of aluminum therein, is used for relatively low power applications. The highly alkaline electrolyte usually consists of NaOH or KOH solution, and yields a higher cell voltage than the neutral electrolyte.
In neutral-pH electrolyte, the cell discharge reaction may be written as:4Al+3O2+6H2O→4Al(OH)3(solid)
In alkaline electrolyte, the cell discharge reaction may be written:4Al+3O2+6H2O+4KOH→4Al(OH)4−+4K+(liquid solution),followed, after the dissolved potassium (or sodium) aluminate exceeds saturation level, by:4Al(OH)4−+4K+→4Al(OH)3(solid)+4KOHIn addition to the above oxygen-reducing reactions, there is also an undesirable, non-beneficial reaction of aluminum in both types of electrolyte to form hydrogen, as follows:2Al+6H2O→2Al(OH)3+3H2(gas).
There is a need for a metal-air battery which can be used as a main power source at locations where electric supply lines do not exist. Such a battery must have a high energy capacity and a high power density and be capable of running for a long period of time under high load. There is also a need for a metal-air battery that can provide much extended “talk time” and “stand-by” time for a mobile phone. A need also exists for a light-weight battery that can power a notebook computer for a much longer period of time (e.g., 12 hours needed to last for a trans-Pacific flight).
State-of-the-art metal-air batteries have exhibited the following drawbacks:                (1) Severe “anode passivation” problem: When the battery is run under high load, large amounts of aluminum hydroxide accumulate on the aluminum anode surface blocking the further access of anode by the electrolyte. Such an anode passivation phenomenon tends to prevent the remaining anode active material (coated or surrounded by a ceramic layer) from contacting the electrolyte. Consequently, the electron-generating function ceases and the remaining active anode material can no longer be used (hence, a low-utilization anode). All metal anodes used in state-of-the-art metal-air batteries suffer from the anode passivation problem to varying degrees.        (2) Severe self-discharge and current leakage problems: “Self-discharge” is due to a chemical reaction within a battery that does not provide a usable electric current. Self-discharge diminishes the capacity of a battery for providing a usable electric current. For the case of a metal-air battery, self-discharge occurs, for example, when a metal-air cell dries out and the metal anode is oxidized by the oxygen that seeps into the battery during periods of non-use. Leakage current can be characterized as the electric current that is supplied to a closed circuit by a metal-air cell even when air is not provided to the cell. These problems also result in a low-utilization anode.        (3) Severe corrosion problem: Four metals have been studied extensively for use in metal-air battery systems: zinc (Zn), aluminum (Al), magnesium (Mg), and lithium (Li). Despite the fact that metals such as Al, Mg, and Li have a much higher energy density than zinc, the three metals (Al, Mg, and Li) suffer from severe corrosion problems during storage. Hence, Mg-air and Al-air cells are generally operated either as “reserve” batteries in which the electrolyte solution is added to the cell only when it is decided to begin the discharge, or as “mechanically rechargeable” batteries which have replacement anode units available. Since the serious corrosion problem of Zn can be more readily inhibited, Zn-air batteries have been the only commercially viable metal-air systems. It is a great pity that high energy density metals like Al, Mg and Li have not been used in a primary or secondary cell.        
Due to their high energy-to-weight ratio, safety of use, and other advantages, metal-air, and particularly zinc-air, batteries have been proposed as a preferred energy source for use in electrically-powered vehicles. However, just like in aluminum-air cells, zinc-air batteries also suffer from the problem of “passivation”, in this case, by the formation of a zinc oxide layer that prevents the remaining anode active material (Zn) from contacting the electrolyte. A number of techniques have been proposed to prevent degradation of battery performance caused by zinc oxide passivation. In one technique, a sufficient (usually excessive) amount of electrolyte was added to allow most of the zinc to dissolve (to become Zn ion, thereby giving up the desired electrons). The large amount of electrolyte added significantly increased the total weight of the battery system and, thereby, compromise the energy density.
In a second approach, anodes are made by compacting powdered zinc onto brass current collectors to form a porous mass with a high surface/volume ratio. In this configuration, the oxide does not significantly block further oxidation of the zinc, provided that the zinc particles are sufficiently small. With excessively small zinc particles, however, zinc is rapidly consumed due to self-discharge and leakage (regardless when the battery is in use or not) and even more serious corrosion problems and the battery will not last long.
An interesting approach to extending the operating time of a battery, particularly a metal-air battery as a main power source for vehicle propulsion, is the use of “mechanically rechargeable” primary battery systems. Such a system normally comprises a consumable metal anode and a non-consumable air cathode, with the metal anode being configured to be replaceable once the metal component therein is expended following oxidation in the current-producing reaction. These systems constituted an advance over the previously-proposed secondary battery systems, which have to be electrically charged for an extended period of time once exhausted, and require an external source of direct current.
Most of these mechanically rechargeable systems are quite complex in construction. For instance, the system disclosed in U.S. Pat. No. 4,139,679 (Feb. 13, 1979 to A. Appelby, et al.) contains an active particulate metal anode component freely suspended in an alkaline electrolyte, and a pump to keep the particulate metal anode in suspension and circulated between air cathodes. After discharge of the metal anode component, the electrolyte is then replaced with an electrolyte containing a fresh particulate metal anode component in suspension.
Mechanically rechargeable metal-air batteries with mechanically replaceable anodes have been further developed, e.g., in U.S. Pat. No. 5,196,275 (Mar. 23, 1993 to Goldman, et al.); U.S. Pat. No. 5,360,680 (Nov. 1, 1994 to Goldman, et al.); U.S. Pat. No. 5,318,861 (Jun. 7, 1994 to Harats, et al.); and U.S. Pat. No. 5,418,080 (May 23, 1995 to Korall, et al.), the teachings of which are incorporated herein by reference. These systems have been designed particularly for use in electric vehicle propulsion, since they facilitate quick recharging of the vehicle batteries simply by replacing the spent anodes, while keeping the air cathodes and other battery structures in place. This mechanical recharging, or refueling, may be accomplished in service stations dedicated to that purpose. However, it is necessary to provide metal-air battery cells that will repeatedly allow insertion and removal of the zinc anode elements for each charge/discharge cycle without causing wear and tear to the mechanically-sensitive air electrode flanking each zinc anode.
Another approach to extending the discharge life of a metal-air battery is the “variable-area dynamic anode” method proposed by Faris (e.g., U.S. Pat. No. 5,250,370, Oct. 5, 1993). Such a battery structure includes electrodes which are moved relative to each other during operation. The electrodes also have areas that are both different in size, with ratios that are variable. The battery structure includes a first electrode which is fixed in a container. A second electrode is moved past the fixed electrode in the container and battery action such as discharge occurs between proximate areas of the first and second electrodes. A third electrode may be provided in the container to recharge the second electrode as areas of the second electrode are moved past the third electrode at the same time that other areas of the second electrode are being discharged at the first electrode. The ratio of the third electrode area to the first electrode area is much larger than 1, resulting in a recharge time that is much faster, thereby improving the recharge speed. However, this battery structure is very complicated and its operation presents a reliability problem.
For a prolonged storage of metal-air cells, Pedicini (U.S. Pat. No. 5,691,074, Nov. 25, 1997) proposed a diffusion-controlled air vent containing isolating passageways that function to limit the amount of oxygen that can reach the oxygen electrodes when the fan is off and the internal humidity level is relatively constant. This isolation reduces the self-discharge and leakage or drain current of the metal-air cells. In U.S. Pat. No. 5,569,551 (Oct. 29, 1996) and U.S. Pat. No. 5,639,568 (Jun. 17, 1997), Pedicini, et al. proposed the use of an anode bag to limit self-discharge of the cell in an attempt to maintain the capacity of the cell. It was stated that, by wrapping the anode in a micro-porous membrane that is gas-impermeable and liquid-permeable, oxygen from the ambient air that has seeped into the cell must go through a solubility step before it can pass through the anode bag to contact and discharge the anode. However, this solubility step is often not a slow step particularly when the oxygen or air ingress rate into the cell is high. This anode bag provides only a moderately effective approach to reducing the self-discharge problem. This is achieved at the expense of making the cell structure very complicated.
Delayed-action or deferred actuated batteries have been proposed for the purpose of prolonging the dry storage period of a battery. For instance, Birt, et al. (U.S. Pat. No. 4,020,247, Apr. 26, 1977 and U.S. Pat. No. 4,185,143, Jan. 22, 1980) disclosed water-activated primary batteries, which could be stored in a dried electrolyte condition and become activated whenever water is added. Similarly, seawater-activated batteries are disclosed by Rao, et al. (U.S. Pat. No. 4,910,104, Mar. 20, 1990; U.S. Pat. No. 5,116,695, May 26, 1992; and U.S. Pat. No. 5,225,291, Jul. 6, 1993). Once water or seawater is added to these cells, all relevant reactions (including those associated with the self-discharge, current drain, passivation, and/or corrosion) will proceed and continue until completion. These batteries, once activated, still suffer from the premature power loss problems due to self-discharge, current leakage, and/or passivation.
Previous attempts to alleviate the anode corrosion problems in a metal-air cell included the addition of corrosion-inhibiting ingredients (e.g., In, Bi, Ca, CaO, PbO, and Bi2O3); e.g., as disclosed in U.S. Pat. No. 5,139,900 (Aug. 18, 1992 to K. Tada, et al.) and U.S. Pat. No. 5,773,176 (Jun. 30, 1998 to J. Serenyi). In U.S. Pat. No. 5,837,402 (Nov. 17, 1998), K. Araki, et al. disclosed an anode that is composed of zinc powder particles having their surfaces coated with copper/indium or silver. However, the coating remains porous so that electrolyte can still contact the underlying zinc particle in order for the zinc particle to still function as an active material. These porous coatings serve to reduce the corrosion problem, but do not isolate the anode active material (zinc) from the electrolyte. Furthermore, they do not provide effective protection against self-discharge or current leakage.
Therefore, it is an object of the present invention to provide an anode material that significantly reduces or eliminates the anode passivation, self-discharge, current leakage, and/or anode corrosion problems.
It is another object of the present invention to provide an anode that has a controlled release function.
Still another object of the present invention is to provide an electrochemical cell that contains a controlled release anode for an extended cell operating life.
A specific object of the present invention is to provide a metal-air battery that has a long operating life.
Another specific object of the present invention is to provide an anode that can make use of high energy density metals such as Li, Mg, and Al by avoiding or alleviating the otherwise severe corrosion problems commonly associate with these metals.